Cooperative inter-vehicular applications usually rely on the exchange of broadcast hop status messages among vehicles which provide detailed information about vehicle speed, position, heading, acceleration or the like. These messages are called beacons and are transmitted for example periodically at a fixed or variable beaconing rate.
One of the problems is, that the aggregated load on the transmission channel, for example wireless channel, due to periodic beacons can rise to a point where it can limit or prevent the transmission of other types of messages. This is called channel congestion due to beaconing activity.
Various control schemes have been proposed to prevent this situation, i.e. channel congestion. A control scheme may for example decrease the beaconing rates or decrease the transmit power, thus reducing the number of vehicles in transmission range of each other or a combination of both of them as disclosed in the non-patent literature of M. Sepulcre, J. Mittag, P. Santi, H. Hartenstein, and J. Gozalvez, “Congestion and Awareness Control in Cooperative Vehicular Systems”, Proceedings of the IEEE, vol. 99, no. 7, pp. 1260-1279, 2011.
For a practical implementation, so-called decentralized schemes have been proposed where no centralized infrastructure is needed for controlling the messages. One of the further beaconing rate control schemes is for example disclosed in the non-patent literature of                J. B. Kenney, G. Bansal and C. E. Rohrs, “LIMERIC: A Linear Adaptive Message Rate Algorithm for DSRC Congestion Control,” IEEE Transactions on Vehicular Technology, vol. 62, no 9, pp. 4182-4197, 2013,        T. Tielert, D. Jiang, Q. Chen, L. Delgrossi and H. Hartenstein, “Design Methodology and Evaluation of Rate Adaptation Based Congestion Control for Vehicle Safety Communications,” Vehicular Networking Conference (VNC), 2011 IEEE, pp. 116-123, 2011,        Y. P. Fallah, C. L. Huang, R. Sengupta and H. Krishnan, “Analysis of Information Dissemination in Vehicular Ad-Hoc Networks With Application to Cooperative Vehicle Safety Systems,” IEEE Transactions on Vehicular Technology, vol. 60, no 1, pp. 233-247, 2011 and        C.-ling Huang, Y. P. Fallah, R. Sengupta, and H. Krishnan, “Adaptive Intervehicle Communication Control for Cooperative Safety Systems,” IEEE Network, vol. 24, pp. 6-13, 2010.        
Although most of them are able to bring the channel load to a desired level, none of them is able to meet global fairness, wherein no vehicle should be allocated arbitrarily less resources than its neighbors under the constraints imposed by the available capacity. In particular most of them provide a very basic notion of fairness and how beaconing rates are allocated.
A further problem is that global fairness among the vehicles is not achieved in multi-hop scenarios where not all vehicles are in the range of each other or a remarkable overhead is introduced in order to meet only an approximate fairness goal. The lower layers of communication stack are very similar in both American and European specifications; in both of them, IEEE 802.11p provides a CSMA-based medium access control MAC and supports frame class priority by enhanced distributed channel access mechanism EDCA. Most of the transmissions are broadcast in nature and use a fixed contention window and no acknowledgement or retransmission. ETSI defines a 10 MHz control channel for vehicular communications at 5.9 GHz as disclosed in the non-patent literature of ETSI EN 302 663, “Intelligent Transport Systems (ITS); Access layer specification for Intelligent Transport Systems operating in the 5 GHz frequency band”, V0.1.3, 2012. Periodic beaconing over one-hop broadcast communications supports cooperative inter-vehicular applications by disseminating status and environmental information to vehicles on the control channel what has been called cooperative awareness service as disclosed in the non-patent literature of ETSI TS 102 637-2, “Intelligent Transportation Systems ITC); Vehicular Communications; Basic Set of Applications; Part 2: Specification of Cooperative Awareness Basic Service”, 2010.
The above mentioned decentralized congestion control has also been published by ETSI in the non-patent literature of ETSI TS 102 687 “Intelligent Transport Systems (ITS); Decentralized Congestion Control Mechanism for Intelligent Transport Systems operating in the 5 GHz range; Access Layer part”, 2011, which can accommodate a variety of controls such as transmit power, message rate or receiver sensitivity. The rate of beacons has an influence on the quality of service of the applications. Since safety-related applications usually need the maximum beaconing rate of 10 beacons/s as disclosed in the non-patent literature of ETSI TS 102 637-2, “Intelligent Transportation Systems ITC); Vehicular Communications; Basic Set of Applications; Part 2: Specification of Cooperative Awareness Basic Service”, 2010, it is expected to be the default one. In a high-density traffic scenario, more than a hundred vehicles may be in range even for moderate transmission ranges, therefore the problem of channel congestion due to beaconing activity has to be taken into account.
Transmit power control (TPC) has been proposed as a mechanism for congestion control by both ETSI standards as disclosed in the non-patent literature of ETSI TS 102 687 “Intelligent Transport Systems (ITS); Decentralized Congestion Control Mechanism for Intelligent Transport Systems operating in the 5 GHz range; Access Layer part”, 2011, and recently in the non-patent literature of M. Sepulcre, J. Mittag, P. Santi, H. Hartenstein, and J. Gozalvez, “Congestion and Awareness Control in Cooperative Vehicular Systems”, Proceedings of the IEEE, vol. 99, no. 7, pp. 1260-1279, 2011, and E. Egea-Lopez, J. J. Alcaraz, J. Vales-Alonso, A. Festag and J. Garcia-Haro, “Statistical Beaconing Congestion Control for Vehicular Networks”, IEEE Transactions on Vehicular Technology, vol. 62, no. 9, pp. 4162-4181, 2013.
However, TPC is prone to instabilities and its accuracy relies on the quality of the propagation estimation. Joint transmit power and rate control may be an option, especially to enforce particular application quality of service requirements, as disclosed in the non-patent literature of M. Sepulcre, J. Mittag, P. Santi, H. Hartenstein, and J. Gozalvez, “Congestion and Awareness Control in Cooperative Vehicular Systems”, Proceedings of the IEEE, vol. 99, no. 7, pp. 1260-1279, 2011, Y. P. Fallah, C. L. Huang, R. Sengupta and H. Krishnan, “Analysis of Information Dissemination in Vehicular Ad-Hoc Networks With Application to Cooperative Vehicle Safety Systems,” IEEE Transactions on Vehicular Technology, vol. 60, no 1, pp. 233-247, 2011 and C.-ling Huang, Y. P. Fallah, R. Sengupta, and H. Krishnan, “Adaptive Intervehicle Communication Control for Cooperative Safety Systems,” IEEE Network, vol. 24, pp. 6-13, 2010.
Regarding generic beaconing rate control proposals, conventional methods as disclosed in the non-patent literature of J. B. Kenney, G. Bansal and C. E. Rohrs, “LIMERIC: A Linear Adaptive Message Rate Algorithm for DSRC Congestion Control,” IEEE Transactions on Vehicular Technology, vol. 62, no 9, pp. 4182-4197, 2013, and T. Tielert, D. Jiang, Q. Chen, L. Delgrossi and H. Hartenstein, “Design Methodology and Evaluation of Rate Adaptation Based Congestion Control for Vehicle Safety Communications,” Vehicular Networking Conference (VNC), 2011 IEEE, pp. 116-123, 2011 propose transmission rate control algorithms to comply with a global generic beaconing rate goal. The former, called LIMERIC, uses a linear control based on continuous feedback (beaconing rate in use) from the local neighbors, whereas the latter, called PULSAR, uses additive increase multiplicative decrease (AIMD) with binary feedback (congested or not) from one and two-hop neighbors. Both of them, however, show several limitations. Regarding fairness, LIMERIC aims at proportional fairness whereas PULSAR at max-min fairness, but none of them define it formally.
LIMERIC is shown to converge to a single fixed point, that is, a unique rate for every vehicle, which is below the optimal proportional fairness rate by design. In fact, there is a trade-off between the convergence speed and the distance to the optimal value. A problem is the fact that the convergence is only guaranteed when all the vehicles are in range, which is clearly unrealistic.
Regarding PULSAR, it is not clear that the usual assumptions used with AIMD in other contexts, such as wired networks or end-to-end congestion, hold in a vehicular scenario, as in particular, the assumption that all users see the same congestion feedback. In addition, a number of modifications to the basic AIMD are described without showing how it may affect fairness. Finally, it requires synchronized updates and piggybacking of two-hop neighbor congestion information.
In the non-patent literature of B. Kim, I. Kang, H. Kim, “Resolving the Unfairness of Distributed Rate Control in the IEEE WAVE Safety Messaging,” IEEE Transactions on Vehicular Technology, to appear, online early access available, 2014 it is shown that both of them actually may fall into unfair configurations and a solution is proposed to compare the target rate with the average rate of the neighbors before applying an AIMD control. However, this only ensures that two neighbor vehicles cannot diverge in their settings but do not ensure correct convergence to a fair configuration in a realistic scenario.
Therefore for a practical implementation a control method should be fair and decentralized: First, vehicles should control their neighbor vehicles and without relying on any centralized infrastructure. Besides, to reduce the signal overhead, the exchanged information should be kept to a minimum. Fairness must be guaranteed as a safety requirement since beacons are used to provide vehicles with an accurate estimate of the state of their neighbors. In general, the higher the beaconing rate, the higher the quality of the state information as disclosed in M. Sepulcre, J. Mittag, P. Santi, H. Hartenstein, and J. Gozalvez, “Congestion and Awareness Control in Cooperative Vehicular Systems,” Proceedings of the IEEE, vol. 99, no. 7, pp. 1260-1279, 2011. Consequently, no vehicle should be allocated arbitrarily less resources than its neighbors, under the constraints imposed by the available capacity. Moreover, global fairness should be achieved, that is, not only among neighboring vehicles but among all vehicles contributing to congestion. Finally, the control should also provide quick and effective adaption to changes in the environment, such as the channel conditions and the number of vehicles in range. The limits on such capabilities are captured by the convergence properties of the algorithm in use. Several beaconing rate control schemes have been proposed in the non-patent literature:                J. B. Kenney, G. Bansal and C. E. Rohrs, “LIMERIC: A Linear Adaptive Message Rate Algorithm for DSRC Congestion Control,” IEEE Transactions on Vehicular Technology, vol. 62, no. 9, pp. 4182-4197, 2013        T. Tielert, D. Jiang, Q. Chen, L. Delgrossi and H. Hartenstein, “Design Methodology and Evaluation of Rate Adaption Based Congestion Control for Vehicle Safety Communications,” Vehicular Networking Conference (VNC), 2011 IEEE, pp. 116-123, 2011        Y. P. Fallah, C. L. Huang, R. Sengupta and H. Krishnan, “Analysis of Information Dissemination in Vehicular Safety Systems,” IEEE Transaction on Vehicular Technology, vol. 60, no. 1, pp. 233-247, 2011        C.-ling Huang, Y. P. Fallah, R. Sengupta, and H. Krishnan, “Adaptive Intervehicle Communication Control for Cooperative Safety Systems,” IEEE Network, vol. 24, pp. 6-13, 2010.        
Although most of them are able to bring the channel load to the desired level, none of them is able to meet all the aforementioned requirements. In particular, most of them provide only a very basic notion of fairness in how beaconing rates are allocated, e.g. without a formal definition and rigorous convergence support. Moreover, either global fairness is not achieved in multi-hop scenarios, when not all vehicles are in range of each other, or a remarkable overhead is introduced in order to meet only an approximate goal. As we shall show in the following sections, when faced with non-trivial (realistic) arrangements of vehicles, they converge to clearly unfair beaconing rate allocations.